1. Field of the Invention
Embodiments of the present invention relate to an optical writing head and an image forming apparatus and, in particular, to an optical writing head which is used for an electrophotographic copying machine, a printer, and a facsimile machine and forms an imaging spot by a lens array projecting beams from a plurality of light emitting elements onto a light receiving surface and an image forming apparatus using the optical writing head.
2. Description of the Related Art
Until now, an optical writing head used for electrophotographic copying machine and other devices includes a light emitting element array in which a plurality of light emitting elements such as a light emitting diode (LED) is arranged, and a lens array in which a plurality of lenses is arranged in parallel between photosensitive drums which are image carriers.
Luminous flux modulated according to an image signal is emitted from a light emitting element and concentrated by the lens array at the surface of the photosensitive drum in a spot shape to record an image.
Such an optical writing head is required to perform higher-definition printing.
The photosensitive drum is cylindrical and is rotated around the axis of the cylinder as a rotation axis. The rotation axis deviates from the center axis of the photosensitive drum due to a production error and an installation error at the time of assembly.
For this reason, an image plane moves back and forth in the optical-axis direction according to the rotation of the photosensitive drum. As a result, a spot position and a spot diameter formed on the surface of the photosensitive drum are varied according to the rotation of the photosensitive drum, which produces uneven density and color change on the recorded image.
Hitherto, U.S. Patent Application Publication US2009/0086328A1 discusses a unit capable of reducing a change in a spot position caused by an image plane moving back and forth in the optical-axis direction by making an image side of a lens array telecentric. According to such a configuration, a principal ray is parallel to the optical axis, so that a change in a spot position is smaller even if the image plane moves back and forth and a change in a spot diameter may be reduced within the focal depth of the lens array.
In order for a conventional method to perform a higher-definition printing, it is required to increase the numerical aperture on the image side (image-side NA) and form a light collective spot to be small in diameter on the surface of a photosensitive drum.
On the other hand, the increase of the image-side NA of the lens array decreases the focal depth of the lens array, which narrows the tolerance of a change in position of the image plane.
Thus, in order to perform a higher-definition printing, the photosensitive drum needs to be accurately installed at the time of production and assembly. This increases the cost of an optical writing head and a photosensitive drum.
Accordingly, the above situation demands an optical writing head in which a light collective spot of a small diameter and a deep focal depth go together.